This invention relates to the powder coating of electrically nonconducting substrates.
Powder coating is a technique used to provide a durable coating on a surface. Powder particles of a curable organic powder-coating compound are electrostatically charged and directed toward the surface of a substrate. When the substrate is a grounded or connected to an oppositely charged metal, the particles are attracted to the surface and adhere to the surface temporarily. The surface is thereafter heated to elevated temperature to cure the curable organic compound to form the final coating.
Powder coating is a preferred alternative to painting or electrophoretic paint coating. In these processes, solvents are used as carriers for the paint pigments and other constituents of the paint coating. The solvents used for high-quality paint coatings include volatile organic compounds (VOCs), which are potentially atmospheric pollutants. Powder coating utilizes no solvents and no VOCs, and is therefore substantially more environmentally friendly.
Powder coating is more difficult when the substrate is an electrically nonconducting material such as a plastic or ceramic. Several techniques have been developed to impart sufficient electrical conductivity to the substrate that it can be electrostatically powder coated. A conductive material such as graphite can be added to the substrate to improve its conductivity, but this technique has the drawback that it requires modification of the character of the substrate. The substrate can be preheated so that the powder particles partially cure and stick when they initially contact the hot surface, but this approach requires that the substrate be heated to temperatures that cannot be tolerated by some types of substrates such as organic-matrix composite materials. In yet another approach, an electrically conductive primer, typically containing metallic or graphite particles, is coated onto the surface of the substrate.
Although this approach is operable, it leaves the finished part with an electrically conductive coating between the substrate and the cured powder coating. This electrically conductive coating can interfere with some uses of the finished part, which otherwise would not exhibit electrical conductivity.
There is a need for an improved approach for electrostatic powder coating of electrically nonconducting objects. Such an approach would find widespread application in the coating of composite materials, ceramics, plastics, and the like. The present invention fulfills this need, and further provides related advantages.